Next, we recall from [1]} the definition of MBM classes.
Remark that any birational map between hyperkähler manifolds
\(\varphi : M\dasharrow M^{\prime }\)  is an isomorphism in codimension one
(in general this easily follows from the nefness of the canonical
class) and therefore induces an isomorphism
on the second cohomology. We say that \(M\)  and \(M^{\prime }\)  are birational
models of each other.
